To Double-space or Not?

There’s someone on Slate, and hence on NPR, who insists that it’s improper to double-space between sentences. Although we were all taught in typing class that we should use two spaces between sentences, that’s only because we were using a mono-spaced typeface; the way real type is set, he said, is with a single space between words and between sentences.

Like, what does he know? Has he ever set type? I have. as early as in Junior High School, I took Print Shop, and was taught: between words, an en space; between sentences, an em space.

“En” space is the space used up by a lower-case n character. “Em” space is that used up by a lower-case m character. Look at them closely: an m takes up twice the space as an n. Note also: an M is twice the space as an N. An M is also called a “quad”, because it is square, or quadrangular.